Fluorescent lamps have heretofore been used most widely as illuminating devices. As compared with incandescent lamps, fluorescent lamps generate a considerably lesser amount of heat but nevertheless release a fairly large amount of heat, so that they are not suited to use in a low-temperature atmosphere, for example, in freezing or refrigerating showcases. They have another drawback in that when installed and used at locations exposed to rain as is the case with advertising sign lamps or in places subject to the influence of water as in refrigerating showcases for fresh fish, the lamp is liable to develop electrical troubles such as electrical insulation failure and improper illumination. Further when the fluorescent lamp is installed and used in a show window or showcase, there is the likelihood that a light blocking member such as lampshade will be needed to shield the eyes of the customer from the illuminating light, which is inconvenient.